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hot water tank as buffer with plate heater supply from wood boiler
ran into an interesting job that had a burnham hydronic 4 section from the late 80s that was replaced under warranty around 2003 and now leaking again. Around the same time as the warranty replacement a wood boiler was added with undersized feeds (about 120 feet of 3/4" pex with no diffusion barrier - ah the good ole days). At least there was a certain presence of mind, if not high awareness on the oxygen infiltration or btu capacity aspects, and the wood boiler supply is run into a stainless plate heat exchanger.
the system side of that exchanger is piped as a zone on the boiler (there were three zones and they combined one and took the third zone for the heat exchanger and that is just tricked to run on the boiler aquastat. And the boiler has a tankless supplying domestic hot water.
the boiler seldom starts in the winter according to oil usage, but i haven't observed operation long enough on a cold day to see if both heat legs are calling and domestic is turn on if and how quickly the boiler kicks in for keep up but i wouldn't be surprised.
so the second burnham is now leaking, haven't had the skin off to see if its a crack or between sections or . . . ( plenty of threads i'm sure to talk about this habit of those boilers) but at least I can't indict potentially oxygenated water from the uncoated leads to the wood boiler because of the plate heat exchanger. Owner is understandably perplexed at cast iron boilers that seem to have service life of condensing boiler and my suggestion is a change of strategy.
I'm thinking to put an indirect tank for the domestic hot water to also act as a buffer tank pulling and storing heat from the plate exchanger a bit like the boiler is now. this guy will eventually repipe with larger barrier leads from the wood boiler and maybe at that point it could be direct without the exchanger but for now I would use that loop for the domestic as a primary loop.
then i would secondary pipe the two heating feeds off the return to the exchanger to protect the domestic tank from seeing lower feed temps because return water from heating loops would cycle through the plate exchanger before hitting the tank again.
so this is adult employee usage of hot water. the biggest difficulty is that with the most heating buffer storage the DHW would be very hot and then at heavier heating loads you would see cooler domestic temps so it would need to be used judiciously or i would need a good variable tempering valve which i have often found is an oxymoron, at least insofar as one that is performing well a few years down the pike compare to new. recommendations?
I also could consider a good flow switch to kill the heating circulators on domestic demand (whatever takagi uses in their on demand heaters seems to be bombproof but i haven't had as much luck finding cost effective standalone, recommendations welcome) or I could use a two setpoint aquastat or two aquastats and set one as a lower limit that kills the heating pumps until the tank temp catches up which would be the equivalent of domestic priority - because i wouldn't want to set the primary as priority, as most of the time there is no problem running the heating loops while the buffer is making up.
finally, the backup function (wood boiler is tended all the time but if it got a leak or needed to be shut down for repairs) /summer DHW I recommended piping a small oil boiler as a secondary on the feed from the exchanger to the indirect DHW tank. as currently piped using the boiler as the buffer, if anything is wrong with the oil boiler or its controls the whole system is down.
So for that reason and as buffer for DHW use I kind of focused on the secondary piping with supply on the feed side and heating demand on the return as a little more predictable in utilizing the buffer and allows domestic primary from a piping standpoint and then using control as a backstop for that. I suppose I could just always run the DHW/system pump constant on when any heating load is on which is pretty much required anyway, i suppose I could use a variable pump for that function and then i'm less likley to get reverse circulation if i piped a more typical feed return piping across the loop. I don't need the secondary setup to avoid low temp return to boiler but it seemed maybe like a decent way to approach this.
please critique away, thanks,